The present invention is directed to a process for dewaxing waxy oil feeds. This invention is particularly directed to a process for dewaxing waxy petroleum oil fractions.
Solvent dewaxing of waxy petroleum oil feeds to obtain lubricating oil stocks includes the step of contacting a cold oil/solvent filtrate stream from a solvent dewaxing process with a selective permeable membrane to selectively separate the cold oil/solvent filtrate into a cold solvent permeate stream and a cold filtrate stream. Part of the cold solvent permeate stream is used to improve unit heat integration and to provide incremental warm dilution. The remainder is recycled to an oil/solvent/wax feed to the wax filtration step. The separated cold filtrate stream is contacted by indirect heat exchange with warm waxy oil feed to cool the warm waxy oil feed.
In solvent dewaxing of petroleum lubricant range hydrocarbons, cold solvent is typically added to a hot waxy raffinate to control crystallization of the wax in the feed. Usually there is a large temperature differential (.DELTA.T&gt;20.degree. C.), which results in non-optimum wax crystallization rate and loss of yield. Chilling of the feed is accomplished by indirect heat exchange against cold filtrate from the dewaxing filters and with refrigerant. Solvent for recycle is usually recovered at large expenditure of energy from the filtrate by a combination of heating, multi-stage flash vaporization, and distillation operations. The hot solvent so recovered is then chilled again at considerable expense to the desired temperature for recycling to the wax filter feed.
In a conventional solvent dewaxing process a waxy oil feed is mixed with solvent from a solvent recovery system. The waxy oil feed/solvent mixture is cooled by indirect heat exchange in a scraped-surface, double pipe heat exchanger against cold filtrate, which is a mixture of oil and solvent recovered from a filter used to separate wax from a wax containing stream. The cold filtrate is a mixture of oil and solvent. The cooled feed mix is injected with additional cold solvent from the solvent recovery system. The resultant mixture is further cooled against vaporizing propane, ammonia, or other refrigerant gas in a second scraped-surface double pipe exchanger. The chilled feed slurry is mixed with more chilled solvent from the solvent recovery system to obtain a filter feed.
The amount of circulating solvent is typically limited by either the capacities of the solvent recovery sections or the capacity of the refrigeration system used to cool the recovered solvent to the desired injection temperatures. These limitations on the solvent availability can restrict the feed rate to the filter since the filter feed (high viscosity oil plus low viscosity solvent) must have a sufficiently low viscosity to achieve an acceptable filtration rate. A circulating solvent temperature significantly different from that of the charge mix at the point of injection can lead to shock chilling.
In current practice, dewaxing of waxy feed is performed by mixing the feed with a solvent to completely dissolve the waxy feed at a suitable elevated temperature. The mixture is gradually cooled to an appropriate temperature required for the precipitation of the wax and the wax is separated on a rotary filter drum. The dewaxed oil is obtained by evaporation of the solvent and is useful as a lubricating oil of low pour point. Accordingly, the dewaxing apparatus is expensive and complicated. In many instances the filtration proceeds slowly and represents a bottleneck in the process because of low filtration rates caused by the high viscosity of the oil/solvent/wax slurry feed to the filter. The high viscosity of the feed to the filter is due to a low supply of available solvent to be injected into the feed stream to the filter. In some cases, lack of sufficient solvent and/or inappropriate injection temperature can result in poor wax crystallization and ultimately lower lube oil recovery.
Use of solvents to facilitate wax removal from lubricants is very energy intensive, due to the requirement for separating from the dewaxed oil and recovery of the expensive solvents for recycle in the dewaxing process. The solvent is conventionally separated from the dewaxed oil by the addition of heat, followed by a combination of multistage flash and distillation operations. The separated solvent vapors must then be cooled and condensed and further cooled to the dewaxing temperature prior to recycle to the process. Serious limiting factors in the conventional solvent dewaxing process are the cost and size of the filters, the cost, size, and operating expense of the distillation equipment needed to separate the solvent from the dewaxed oil, and the cooling apparatus and cooling capacity required to cool the warm solvent separated from the dewaxed oil. The filter capacity could be increased if there were available more solvent by simply further diluting the oil/solvent/wax mixture feed to the filter to lower the viscosity of the feed, and if crystallization were better controlled. However, increasing the amount of solvent available to dilute the feed to the filter requires increasing the means of heating and separating solvent from dewaxed oil and increasing the cooling capacity to cool the separated warm solvent prior to recycle.
Problems to be solved include increasing the amount of solvent available to the solvent dewaxing process without increasing the overall solvent inventory and without increasing the size and capacity of the oil/solvent recovery distillation system and the refrigeration capacity required to cool the warm solvent separated by distillation and to simultaneously provide pre-dilution solvent at higher temperature. An additional problem is to increase the filtration capacity of the process without providing additional filtration apparatus.
It is an important object of the present invention to increase the amount of dewaxing solvent available to the dewaxing process by increasing the rate of recycle of the solvent to the process. Particularly, the ratio of internal circulating solvent to solvent recovered by vaporization from oil product can be increased to more than 3:1 Accordingly, it is another object of the present invention to increase the amount of dewaxing solvent available to the dewaxing process without increasing the distillation capacity of the oil/solvent recovery system and without increasing the refrigeration capacity required to cool to the dewaxing temperature warm solvent recycled from the oil/solvent recovery system to the dewaxing process.
It is another object of the present invention to utilize a selective permeable membrane to contact cold oil/solvent filtrate from the filter to selectively separate the cold filtrate into a cold solvent permeate stream and a cold filtrate retentate stream which contains the dewaxed oil and the remaining solvent and to recycle the cold solvent permeate stream to the filter feed stream. Yet another object is to increase the oil/solvent/wax slurry filtration rate by increasing the solvent recycle to the filter feed stream and decreasing the viscosity of the oil/solvent/wax slurry feed to the filter.
A further object is to reduce the required distillation capacity of the solvent recovery distillation operations. Another object is to increase filter rate by controlling wax crystal growth. A significant advantage of this invention is minimizing membrane area.